Total organic carbon (TOC) analysis is one of the most critical tests performed in high purity water systems used in the pharmaceutical and semiconductor industries. TOC is a measure of the total carbon present excluding all inorganic carbon in the form CO2, HCO3−, or CO32−. Stringent sub μg/L restrictions have been set for ultra-pure water (UPW) by the International Technology Roadmap for Semiconductors (ITRS) and the Semiconductor Equipment and Materials International (SEMI) due to the deleterious effect of even trace organics on production. Additionally, SEMI F63 stipulates that any TOC analyzer should have a limit of detection of 50 ng/L or less. Pharmaceutical TOC, however, is less critical, requiring water for injection to be less than 500 μg/L carbon according the United States and European Pharmacopeias, and any method used for TOC must not have a limit of detection above 50 μg/L carbon.
Because TOC is a measure of all organic compounds present, no single technique is adequate to measure the compounds directly due to the range of chemistries present. For this reason, nearly all TOC analyzers are dependent upon the indirect measurement of carbon species following oxidation of the organics and measurement as CO2. The two primary methods of determination of the produced CO2 are non-dispersive infrared absorption spectroscopy (NDIR) and conductivity. While these methods are generally effective, each requires digestion of the sample prior to analysis.
Absorption spectroscopy offers an alternative that does not require digestion prior to analysis. In the deep ultraviolet (UV) light, all compounds absorb to some degree. Further, in general, absorption by an overwhelming majority of compounds increases monotonically at wavelengths below 220 nm and hence the sensitivity increases with decreasing wavelength. The smaller the probe wavelength, the smaller the particles that can be detected by scattering. Absorption methods have previously been used to measure TOC in a variety of matrices such as waste waters, natural, fresh, and sea waters, and purified water streams. Unfortunately, current commercially available equipment has a low operating limit of 0.1 mg/L, which does not even meet the mandated limit of detection (LOD) for pharmaceutical waters, much less the LOD of the semiconductor industry.
The LOD of absorption spectroscopy can be increased by increasing the path length of the absorbance cell to increase its sensitivity. Cavity-enhanced techniques can be used to increase the effective path length thereby lowering the limit of detection without requiring larger physical paths. In cavity-enhanced absorption spectroscopy (CEAS), the absorbance cell has reflective surfaces so that light bounces back and forth across the same path multiple times before detection. To date, CEAS has primarily been applied to samples in the gas phase and has shown only limited promise for use on liquids.
In view of the above discussion, it can be appreciated that it would be desirable to have an effective system and method for performing cavity enhanced absorption spectroscopy on liquids.